In the art, there are known methods and systems for determining the data rate of a received signal. Communication standards often address problems of variable bandwidth due to interference reflections, speech activity controlling and the like, by providing several modes of transmission, each in a predetermined data rate.
The definition of the basic terms used hereinbelow can be found in "CDMA--principles of spread spectrum communication", by A. J. Viterbi, Addision-Wesley Publishing Company, 1995.
Reference is now made to FIG. 1 which is a schematic illustrating of a system, referenced 1, known in the art. System 1 receives an incoming signal, which has been transmitted at a rate, selected from a set of rates. In the present example, the rate set includes four rates. The rates are, marked 1, 1/2, 1/4 and 1/8. The maximal volume of information which can be transmitted in rate 1/2 is half the maximal volume of information which can be transmitted in rate 1. The same applies for rates 1/4 and 1/2, and rates 1/8 and 1/4, respectively.
System 1 includes a plurality of decoding and estimating units 2, 4, 6, and 8 for each available rate. Unit 2 includes a decoder 10 and a reception quality estimator 12, which is connected to the decoder 10. Unit 4 includes a decoder 14 and a reception quality estimator 16, which is connected to the decoder 14. Unit 6 includes a decoder 18 and a reception quality estimator 20, which is connected to the decoder 18. Unit 8 includes a decoder 22 and a reception quality estimator 24, which is connected to the decoder 22.
System 1 further includes a decision controller 26 which is connected to units 2, 4, 6 and 8.
The received encoded signal is provided to each of the decoding and estimating units 2, 4, 6, and 8. The decoder 10 decodes the encoded signal according to rate=1 and provides the decoded signal to the reception quality estimator 12. The reception quality estimator 12 processes the decoded signal so as to produce a quality value and provides it to the decision controller 26.
The decoder 14 decodes the encoded signal according to rate=1/2 and provides the decoded signal to the reception quality estimator 16. The reception quality estimator 16 processes the decoded signal so as to produce a quality value and provides it to the decision controller 26. The same process is executed by units 6 and 8 so as to produce quality values for rates 1/4 and 1/8, respectively.
Then, the decision controller 26 selects the rate which has best quality value and informs the receiver that the received encoded signal is to be processed according to the selected rate. Such systems are described in several U.S. patents.
U.S. Pat. No. 5,230,003 to Dent et al describes a method which decodes a received signal according to two rates, thereby producing two decoded samples, provides a quantitative measurement for each of these decoded samples, compares between the quantitative measurements and selects a rate accordingly.
U.S. Pat. No. 5,566,206 to Butler et al describes a method which decodes a received signal according to plurality of rates, re-encodes all of the decoded samples, counts the number of symbol errors, combines quality estimation and selects the rate accordingly.
U.S. Pat. No. 5,671,255 to Mao Wang et al describes a method which decodes a received signal according to a plurality of rates, calculates detection statistic according to several parameters, such as CRC, symbol error rate and the output quality value of the Viterbi decoder and selects a rate accordingly.
U.S. Pat. No. 5,638,408 to Takaki describes a method which decodes a received signal according to a plurality of rates, compares the four path metric values and selects a rate accordingly.
U.S. Pat. No. 5,509,020 to Iwakiri et al describes a method which decodes a received signal according to a plurality of rates, compares the path metric of the decoder at each rate and selects a rate accordingly.
It will be appreciated by those skilled in the art that decoding a received encoded signal according to all possible rates requires a considerable amount of processing power, thereby increasing power consumption. This is extremely crucial in mobile communication systems such as cellular telephones, since these systems have limited power sources.
IS-95 communication standard (CDMA) provides two sets of communication rates. Rate set 1, which includes 1200, 2400, 4800 and 9600 bits per second and rate set 2, which includes 1800, 3600, 7200 and 14400 bits per second.
Each CDMA information frame can be transmitted at any rate in a given rate set. The present example relates to rate set 1. It will be appreciated by those skilled in the art that an IS-95 frame does not include any specific information indicating the rate, according to which it was transmitted.
A receiver receiving this frame has to estimate the correct rate from the received data incorporated therein, in order to correctly decode the information bits, of that frame. According to the standard, the information bits are encoded in a convolutional encoder, having a rate of 1/2. Then, they are subjected to symbol repetition, depending on the rate.
TABLE 1 ______________________________________ Rate name 1/8 1/4 1/2 1 ______________________________________ 1200 2400 4800 9600 - information bits 16 40 80 172 CRC bits 0 0 8 12 encoder tail bits 8 8 8 8 - 24 48 96 192 - repetitions (rpt) 8 4 2 1 transmitted symbols N.sub.s 384 384 384 384 ______________________________________
As will be appreciated by those skilled in the art, the number of transmitted symbols N.sub.s in each frame is three hundred and eighty four. This number is independent of the rate used for transmitting this frame.
The symbols are arranged in an interleaved format and multiplied first by the user's unique Walsh function, and then by a complex pseudo random (PN) chip sequence. Each symbol is multiplied by sixth four chips, each having a real component I.sub.c and an imaginary component Q.sub.c, wherein I.sub.c and Q.sub.c are independent. A complex combination of the real component I.sub.c and the imaginary component Q.sub.c, given by I.sub.c +jQ.sub.c, represents each chip. The complex output signal, which is a result of the multiplication, is then QPSK modulated and transmitted to the channel.
It will be appreciated by those skilled in the art that although the above method provides reasonable rate detection results, the method includes a tremendous amount of calculating operations. Hence, an implementation of this method in software requires high speed processing capabilities, which results in an increased power consumption. An implementation of this method in hardware requires a great number of hardware elements, which are difficult to manufacture and utilize in small size systems.